A principle objective of the Program Announcement to which this grant proposal is responsive is to "support research to improve physical activity measurement through improved instruments, technologies, or statistical/analytic techniques" for use in "general and diverse populations." The work proposed under this exploratory/developmental R21 grant application will involve the validation of a new Physical Activity Monitor (PAM) that was recently developed as a collaborative effort by the University of Virginia and Barron Associates, Inc. Novel features of the PAM device include: (1) the ability to collect inertial sensor and heart rate measurement data simultaneously in a single, discreet, low-profile device; and (2) measurement of the six degree-of-freedom (6-DOF) body-axes accelerations of the wearer's center of mass (CM), which allows complete discrimination of the translational and rotational motions of their CM in three-dimensional space. This capability translates into more accurate activity energy expenditure estimates than is possible with other commercial actigraphs. The PAM system also enables accurate tracking of the timing, frequency, and intensity of physical activity. Both raw and processed data can be stored in PAM system memory for subsequent off-line uploading to a PC or personal digital assistant (PDA), or alternatively uploaded on-line in real time, via Bluetooth wireless communications. Focus of the proposed Year 1 effort will be to validate the accuracy of the PAM energy expenditure estimates and time spent at different activity intensity levels against a metabolic measurement system in a retrospective study of 90 healthy adults. Precision of the PAM system (i.e., inter- and intra-instrument reliability) will also be assessed in the Year 1 effort. The proposed Year 2 effort will involve the prospective validation of the PAM system against the metabolic measurement system in a similar subject population, and comparison of PAM estimates with that of other "professional-grade" activity monitors that are suitable for field use. Physical activities on which subjects will be tested range from simulated activities of daily living, to "naximal aerobic effort. The PAM system has strong potential to impact adverse health effects of physical inactivity, such as cardiovascular disease, obesity and type 2 diabetes. The PAM system will also be of benefit in sleep studies gait studies, pain management, geriatric stability assessment and fall monitoring, and diagnosis of ADHD, to name lust a few applications.